XNA
works with keyboards as well as with gamepads. You might be surprised
to learn that you can plug a USB keyboard into an Xbox 360 and use it
just as you’d use the keyboard on the PC. If you want the program to
work with the keyboard, you can add code that does this, as shown here:
KeyboardState keys = Keyboard.GetState();
if (keys.IsKeyDown(Keys.R)) redIntensity++;
if (keys.IsKeyDown(Keys.B)) blueIntensity++;
if (keys.IsKeyDown(Keys.G)) greenIntensity++;
if (keys.IsKeyDown(Keys.Y))
{
redIntensity++;
greenIntensity++;
}
Note that the process is very similar to how the gamepad works, but there are slight differences. You don’t need to tell the GetState method on the Keyboard which keyboard to read because XNA supports only a single keyboard. The KeyboardState
item that is returned from the call is not actually a piece of paper;
instead, it is an object that provides methods that the program can use
to discover whether a particular key is pressed. Rather than seeing if
the state of a button is set to the value ButtonState.Pressed, the program can call the method IsKeyDown. You supply the IsKeyDown method with a parameter that identifies the key you are interested in, as follows:
if (keys.IsKeyDown(Keys.R)) redIntensity++;
This code is a conditional statement that increases the value of redIntensity if the R key is pressed. The method IsKeyDown returns true if the key is down and false if not. You can, therefore, use it to control the update of the redIntensity value.
1. Stopping the Game with the Escape Key
The Update
method that is created when you make a new XNA game contains a test
that checks the for the Back button on gamepad 1 and calls the Exit
method to stop the game when the Back button is pressed. If you are
using a keyboard instead of a gamepad you will not be able to press this
button to stop the a game. You can add a test for the Escape key on the
keyboard. This key is a "control" key, in that it does not actually
relate to a printable character, but is designed to signal an action you
want the program to take. Other control keys include the Enter key and
the Backspace key. You can use the same IsKeyDown method to test for the Escape key.
if (keys.IsKeyDown(Keys.Escape)) Exit();
This code stops the game when the Escape key is pressed.
2. Using a Gamepad and a Keyboard at the Same Time
If you want to use a gamepad and a keyboard simultaneously, you have to test for both. This means that the Update method now looks like this:
protected override void Update(GameTime gameTime)
{
GamePadState pad1 = GamePad.GetState(PlayerIndex.One);
if (pad1.Buttons.Back == ButtonState.Pressed) Exit();
if (pad1.Buttons.B == ButtonState.Pressed) redIntensity++;
if (pad1.Buttons.X == ButtonState.Pressed) blueIntensity++;
if (pad1.Buttons.A == ButtonState.Pressed) greenIntensity++;
if (pad1.Buttons.Y == ButtonState.Pressed)
if (pad1.Buttons.B == ButtonState.Pressed) redIntensity++;
{
redIntensity++;
greenIntensity++;
}
KeyboardState keys = Keyboard.GetState();
if (keys.IsKeyDown(Keys.Escape)) Exit();
if (keys.IsKeyDown(Keys.R)) redIntensity++;
if (keys.IsKeyDown(Keys.B)) blueIntensity++;
if (keys.IsKeyDown(Keys.G)) greenIntensity++;
if (keys.IsKeyDown(Keys.Y))
{
redIntensity++;
greenIntensity++;
}
base.Update(gameTime);
}
This
code is not good because you are doing the same thing twice, just
triggered in a different way. The Great Programmer, if she ever saw
this, would not be impressed. Fortunately C# provides a way that a
program can combine two conditions and then perform some code if either
condition is true. This way of combining conditions is called the OR
logical operator because it is true if one thing or the other is true,
and it is written in the program as two vertical bars (||):
GamePadState pad1 = GamePad.GetState(PlayerIndex.One);
KeyboardState keys = Keyboard.GetState();
if (pad1.Buttons.B == ButtonState.Pressed ||
keys.IsKeyDown(Keys.R)) redIntensity++;
The OR
logical operator is placed between two Boolean expressions that can be
either true or false. If one or the other expression is true, the
combined logical condition works out to be true. In this code, if the
red button is pressed on the gamepad or the R key is pressed on the keyboard (or both), the redIntensity
value increases. This is exactly what you want, and it means that Color
Nerve can now be played with the gamepad or the keyboard (or both at
the same time). Logical operators are so called because they produce
logical rather than numerical results. There are other logical operators
that you will use as you create more complex programs.
Note:
If you find this
logical operator stuff hard to understand, just go back to the problem
that you are trying to solve. You want the program to perform a
statement (redIntensity++) if the red key is pressed on the gamepad or if the R key is pressed on the keyboard. So you use the OR operator (||) to combine the two tests and make a condition that triggers if one or the other condition is true.
Note:
The sample project
in the directory "02 Color Nerve" in the resources for this article implements the game. You can adjust the colors of the screen by pressing
the gamepad buttons or a key on the keyboard.
